• Conformal Coatings – State of the Art and an Outlook into the Future


    Due to increasing demands placed on electronics worldwide, the requirements for the components that are necessary for the construction of electronics are also more and more stringent. For this reason, conformal coatings, too, are subject to constant re-evaluation. Electronics suppliers are searching solutions likely to meet not only today’s high demands, but also those of future applications. In the following, we will explain why we at Peters consider different types of conformal coatings to be viable for the future, and why, even for a company having more than 50 years of experience in the field of special coatings for electronics, it is a challenge to develop the “jack-of-all-trades”. Today, solvent-based coatings are dominating the conformal coatings market. One can distinguish between two major groupsof solv ent-based coatings: The physically drying acrylic coatings on the one hand, and the oxidative-curing modified alkyd coatings on the other. With these two types of conformal coatings, it is possible today to meet almost all requirements, although the modified alkyd lacquers often reach their limits when it comes to moisture
    insulation resistance, for example in the field of e-mobility. In the case of acrylic coatings, the comparatively low solids content which is due to the solubility of the polymer and the
    resulting high solvent content are opposed to VOC guidelines (VOC = volatile organic compounds) that are increasingly important all over the world. The thermoplasticity, which is also a result of the polymers used, leads to a too quick softening of the material at temperatures beyond 100 °C. This significantly reduces the high stress loads in the temperature shock test. We have presented the main advantages and disadvantages below. Oxidative curing conformal coatings can be regarded as all-round coatings that are tolerant in processing. Their serious disadvantage is certainly to be seen in the relatively long and uncontrolled cross-linking reaction i.e. oxidative curing, the duration of which depends on the temperature and the layer thickness. Since the cross-linking reaction does not take place to the same extent at all points, different cross-linking states are usually present on the assembly. In this context, it is particularly critical that in the event of excessive layer thicknesses, a permanent gel-like state can occur at the potentials under the dried ink surface. This would significantly reduce the protective effect
    of this coating, creating the risk of e-cor-rosion under the coating. A not really new class of conformal coatings is re-emerging with the synthetic rubber genre: These solvent-based
    systems are characterised by excellent electrical insulation values and an exceptional flexibility. Thanks to the low Young modulus at low temperatures, these systems are an ideal solution for
    sensitive components. A low solubility in various solvents leads to a very low solids content. However, due to the polymer structures, only a low resistance against

    greases and oils can be reached.Ther e is a noticeable difference between the solvent-based and solvent-free conformal coatings when it comes to covering the edges of component legs. The evaporation of solvents in the drying process causes volume shrinkage in such a way that a cavity would form under both the components and the component legs. When using solvent-free conformal coatings, however, no significant volume shrinkage occurs, so that micro-potting is more appropriate for the components than coating. This effect has far-reaching consequences. Due to the complete encapsulation there is, on the one hand, a much higher protection than with classic conformal coatings, while on the other, given the high coating thicknesses,the effects of the different thermalexpansion coefficients can no longer be neglected. With a special class of conformal coatings available in the market that feature thixotropic properties, it is possible to achieve, through appropriate processing, a better edge coverage than with conventional solvent-based conformal coatings. The term thixotropy describes the very common effect that gels liquify when subjected to shear stress (e.g. when stirred or shaken). At the end of this action, they would solidify again, and their viscosity will change again. Coatings with thixotropic properties become slightly liquid during application and then thicken quickly. The best example of a thixotropic liquid is ketchup: One must shake the ketchup bottle to liquefy the ketchup.

    In electronics, both slightly thixotropic ink variants and highly thixotropic gels are more and more common. One would choose a slightly thixotropic ink variant whenever a two-dimensional
    application with a homogeneous surface along with an optimum edge coverage has been specified. High-thixotropic gels are mainly chosen in electronics as the so-called “dam & fill” variant where the gel is arranged in sharp contours to build a “dam” that is subsequently filled with a well-flowing ink system. In other applications, the gel permits to separate very precise coating areas from those to be left out. The most common application method, however, is to apply a “dam” in the area of connectors in order to seal them off from the non-flowable “dam” and then coat the assembly over the entire surface with a conformal coating flowing well. The thixotropic conformal coating systems including gels are already available among the further developed acry-
    lates, synthetic rubbers and also as UV-curing conformal coatings.

    2-component conformal coatings

    Since conformal coatings, apart from their moisture protection effect, ensure further protective functions on assemblies, it is becoming more and more reasonable to consider choosing 2-com-
    ponent conformal coatings. Both high chemical resistances and high dielectric or thermally conductive end properties can be particularly well developed within 2-component systems. It is
    possible to achieve high chemical resistances by mixing 2-component systems prior to their application. 2-component conformal coatings can be based on various resins such as polyurethane, epoxy or silicones, although epoxies tend to play a minor role in conformal coating applications due to their high hardness and brittle-ness. Polyurethanes can be formulated in many variants; they offer a good mixture of positive end properties in terms of resistance and elasticity, along with high electrical resistance values. Silicones are even more elastic than polyurethanes while providing high resistances and good electrical end properties. 2-component silicone-based conformal coatings are also particularly suitable for an application on LED assemblies (low
    yellowing). 2-component conformal coating systems offer a variety of options for replacing oxidative conformal coatingsystems. One important benefit is the significantly faster and defined drying of such 2-component systems, no matter whether solvent-based or solvent-free. When taking a look at solvent-based acrylic coatings, one would find that the electrical and mechanical parameters meet most of today’s requirements. However, two weak points have been identified: One is thermoplasticity, and the other is the low layer thickness that can be applied. The thermoplasticity of physically drying coatings directly depends on the molecular size. It cannot be increased significantly, as this would sharply increase the viscosity or reduce the solubility significantly, so that no major enhancements can be achieved. In view of significantly increasing the coating thickness, there are the options of a reduced travel speed or a double
    coating, which in return, however, have an immediate negative effect on productivity, in such a way that they can be implemented only in very few cases. One solution already available is that of using 2-component coatings. As from their chemical and physical properties, 2-component polyurethane (PUR) coatings are most suitable. Polyurethanes are formed through polyaddition reactions between (poly)-isocyanates and polyalcohols. The physical properties of the crosslinked polyurethanes are variable over a wide range. Depending on the type of the starting polymers, either hard, tough or soft-elastic films are produced. In this context, reference is made to the 2-component polyurethane coatings that are based on acrylate resins. The classification of 2-component coatings as 2-component PUR coatings is important and useful, but still insufficient. Therefore, we want to give a brief explanation of the grouping of 2-component PUR chemistry. Among the hardeners, one can distinguish between four groups: The first group relates to the MDI hardeners (methylene di-isocyanates) that are familiar from casting compounds. Because of their toxicity – they have been classifed as toxic according to European labelling (skull and crossbones symbol) – and also their carcinogenicity, they should no longer play a role in conformal coatings in the future.The IPDI-type hardeners (isophorone di-isocyanate) do not fulfil the mechanical requirements sufficiently as regardselasticity and flexibility. The group of TDI hardeners (toluene diisocyanate) is characterised by flexibility. Due to their structure, the yellowing stability is lower than that of HDIs. The HDI (hexamethylene diisocyanate) harden-
    ers are dominating among industrial and automotive coating materials on the one hand, while they fulfil the special requirements of electronic coating very well on the other. Special acrylate resins having a potential to react with polyisocyanates promise a combination of the well-known good electrical properties along with low thermoplasticity, and above all a higher solid
    content. The latter enables significantly higher layer thicknesses of > 50 %.

    UV technology

    As mentioned in the course of this article, solvent-free UV conformal coatings of the Twin-Cure principle are already present in the market; nevertheless, there is still potential for improvement, especially with regard to future requirements. In particular, the electrical moisture insulation properties at high temperatures beyond 80 °C and moisture contents of 80 % RH or higher could be enhanced to comply with increasing insulation requirements. In addition, coating systems from the UV-LED curing field will gain in importance in the future, as this would bring further advantages such as the elimination of UV-C curing (no ozone developed). 1-component UV-LED-curable conformal coatings are already available. Compared to conformal coatings cured by the conventional UV technology, these actually show better results in the area of electrical and mechanical end properties.
    Solvent-free silicone-conformal coating systems

    Due to their special properties, silicone-based systems are considered to beunique; in general, they are chosen forspecial applications such as high-tem-perature or lighting industry applications. Silicone-based systems are available bothas solvent-free, thermally curing coating systems such as our ELPEGUARD DSL 1705, or as solvent-free systems curing at room temperature, as for example our thick-film coatings of the series ELPEGUARD DSL 1706. Our DSL 1705 FLZ, a 1-component solvent-free conformal coating, is listed by UL as a conformal coating in accor-
    dance with UL 746 E. Its application as a thick-film coating is possible and easy to implement, thanks to its viscosity. The curing time is short, it needs only 10 minutes of curing at 110 °C.
    Our conformal coatings DSL 1706 FLZ, DSL 1706 HV-FLZ and DSL 1706 NV-FLZ are single-component, solvent-free Permanent Coatings tested according to UL 94 V-0. Given the different viscosities, it can be applied both in thin or in thick layers. The short curing time of 20 minutes at room temperature can be further shortened by the application of heat. A combination of a silicone conformal coating and the UV-curing feature would then bring about solvent-free, UV-curing silicone conformal coating systems such as our ELPEGUARD Twin Cure DSL 1707 FLZ. This new conformal
    coating variant developed by Peters combines the flexibility of silicones and rapid curing through UV radiation within seconds.Besides its excellent chemical resistance and outstanding
    physical properties, the silicone structure of this thick-film coating ensures a very high permanent temperature resistance of approx. 180°C. This is due to the higher binding energy
    of the Si-O-Si base structure compared to the conventional C-C-C structure of a “non-silicone” coating (approx. 468 kJ/ mol as against 343 kJ/mol).Even at temperatures as low as -50 °C, silicones are characterised by their almost constant elasticity. Silicones «play out» this property in extreme thermal shock tests: Twin-Cure® DSL 1707 FLZ withstands temperature shock loads between -40 and +180°C of a few seconds without damage, even in higher layer thicknesses. On uncleaned assemblies, it would show no cracking on solder paste residues. The electrical insulation properties of Twin-Cure® DSL 1707 FLZ are at a very high level even when exposed to high temperatures and high relative humidity: Even at a load of 85 ° C and 85% relative humidity, the insulation values do not fall below 1000 MOhm. This high climatic strength, combined with a natural low flammability of silicones, is the prerequisite for an approval according to UL 746E even in high layers.
    • Thick film coating ELPEGUARD DSL 1705 FLZ
    • Thick film coatings of the series ELPEGUARD® DSL 1706
    • Thick film coatings of the series ELPEGUARD® Twin-Cure®
    DSL 1707 FLZ
    In summary, this results in five different conformal coating
    systems, the so-called “BIG FIVE” of conformal coatings, which
    meet both today’s and future requirements:
    1. Modified acrylates
    2. Synthetic rubbers
    3. 2-component conformal coatings
    4. UV (LED)-curing conformal
    5. Silicones crosslinking at room temperature, those crosslinking at high temperature and those curing by UV radiation As a conclusion, one can say that in view of increasing require-
    ments placed on conformal coating systems, the way of responding to them will depend on the application. Only a diversity of conformal coating systems can provide a solution here. Both 1-component and 2-component systems based on different technologies such as conventional, solvent-containing and thermally curing systems, plus those of the UV curing field, whether solvent-free, based on silicone or silicone-free, will be referred to in the future as coatings for protecting electronics. Oxidative curing conformal coating systems will play a minor role in the future due to more and more limitations and a progressively restrictive legislation in the area of mandatory labelling. Wherever possible, UV (LED)-curable, solvent-free conformal
    coatings will quickly become established.

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